Lockable overshot

ABSTRACT

Lockable overshot ( 10 ) includes an elongated body ( 12 ), lifting dogs ( 14   a   , 14   b ) and a locking sleeve ( 16 ). Ends ( 18 ) of the lifting dogs ( 14 ) are configured for latching a conventional spearhead point therebetween. The locking sleeve ( 16 ) is retained on the body ( 12 ) and is provided at one axial end with a circumferential wall ( 66 ) and two diametrically opposed gaps ( 68 ). The locking sleeve ( 16 ) is rotatable on the body ( 12 ) between a locked state preventing ends ( 18 ) of the lifting dogs from pivoting away from each other to release a previously latched spearhead point, and an unlocked state where the locking sleeve ( 16 ) allows the lifting dogs ( 14 ) to move so that the ends ( 18 ) can be pivoted away from each other to release a previously latched spearhead point. In this regard, the locking sleeve ( 16 ) is provided at one axial end with a circumferential wall ( 66 ) provided with a pair of diametrically opposed gaps ( 68 ). When the locking sleeve ( 16 ) is in the locked state, the wall ( 66 ) is disposed inside or between ends ( 56 ) of the lifting dogs ( 14 ). This prevents the ends ( 18 ) of the lifting dogs from pivoting away from each other. When in the unlocked state, the gaps ( 68 ) register with the ends ( 56 ) of the lifting dogs ( 14 ) so that the ends ( 56 ) can be pivoted toward each other thereby pivoting the ends ( 18 ) away from each other to release a previously latched spearhead point.

FIELD OF THE INVENTION

[0001] The present invention relates to a lockable overshot, typically,though not exclusively, for use in a wireline spearhead system forlowering and retrieving core barrels and other tools and equipment froma downhole location.

BACKGROUND OF THE INVENTION

[0002] A typical overshot comprises a tubular body in which a pair oflifting dogs is pivotally mounted. The lifting dogs pivot about a pinwhich attaches the lifting dogs to the body. A first end of the liftingdogs is adapted for catching a spearhead point therebetween. A spring isdisposed between the second opposite ends of the lifting dogs forbiasing the first ends toward each other to a capture position where aspearhead point can be caught. The bias of the spring holds the firstends of the lifting dogs together about a caught spearhead point.

[0003] In order to release a captured spearhead point an operator pushesthe second ends of the lifting dogs together against the bias of thespring thereby spreading the first ends of the lifting dogs andreleasing the spearhead point. For safety and efficiency reasons it iscritical that the lifting dogs do not accidentally or prematurelyrelease a caught spearhead point. For example when retrieving a corebarrel from a hole using an overshot, serious injury or death can occurto an operator if the core barrel becomes accidentally released from theovershot while the overshot is being suspended near the top of a drillrig. Similarly, very expensive equipment can be lost downhole if itbecomes detached from an overshot.

SUMMARY OF THE INVENTION

[0004] It is an object of the present invention to provide a lockableovershot in which the possibility of the lifting dogs accidentally orprematurely releasing a caught spearhead point is reduced.

[0005] According the present invention there is provided a lockableovershot including at least: an elongated body,

[0006] at least two lifting dogs pivotally mounted to said body, saidling dogs having respective first ends configured for latching aspearhead point therebetween, said first ends pivotally moveable towardeach other to a latched position wherein a spearhead point is latchedtherebetween and pivotally moveable away from each other to a releaseposition where a latched spearhead point is released; and,

[0007] a locking sleeve retained on said body and switchable between: alocked state where said locking sleeve locks said lifting dogs in saidlatched position and, an unlocked state where said locking sleeve allowssaid lifting dogs to move to said release position.

[0008] Preferably said lifting dogs have respective second ends oppositesaid first ends and said lifting dogs are pivotally mounted at alocation intermediate said first and second ends to said elongated body,and at least a portion of said locking sleeve is disposed inside saidsecond ends when in said locked state.

[0009] Preferably said locking sleeve is rotatably retained on saidelongated body and is switched between said locked state and saidunlocked state by rotating said locking sleeve about a longitudinal axisof said elongated body.

[0010] Preferably said locking sleeve is provided at one axial end witha circumferential wall formed with a gap for each of said lifting dogswherein when in said locked state said wall is located inside saidrespective second ends and when in said unlocked state said second endsare located in said gaps.

[0011] Preferably said lockable overshot further includes détente meansfor releasably holding said locking sleeve selectively in said lockedand unlocked states.

[0012] Preferably said détente means includes a biased locking ballextending from an outer surface of said elongated body and first andsecond recesses formed in said locking sleeve for partially receivingsaid locking ball, said first recess disposed to register with saidlocking ball when said locking sleeve is in said locked state and saidsecond recess disposed to register with said locking ball when saidlocking sleeve is in said unlocked state.

[0013] In one embodiment said circumferencial wall is of constant outerdiameter.

[0014] In an alternate embodiment said circumferential wall is formedwith a reducing outer diameter in a direction toward said lifting dogs.

[0015] Preferably said locking sleeve is further provided with ashoulder of constant outer diameter adjacent said circumferential wallon a side distant said lifting dogs.

[0016] Preferably said first and second recesses are in the form ofaxially extending slots.

[0017] Preferably said lockable overshot further includes bias means forbiasing said locking sleeve axially in a direction toward said liftingdogs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] An embodiment of the present invention will now be described byway of example only with reference to the accompanying drawings inwhich:

[0019]FIG. 1 is an exploded view of an embodiment of the lockableovershot;

[0020]FIG. 2 is a schematic representation of the lockable overshot whenin a locked state;

[0021]FIG. 3 is a representation of the lockable overshot when in anunlocked state; and,

[0022]FIG. 4 is a schematic representation of a second embodiment of thelockable overshot.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0023] Referring to FIGS. 1-3 of the accompanying drawings, the lockableovershot 10 includes an elongated body 12, lifting dogs 14 a and 14 b(hereinafter referred to in general as “lifting dogs 14”) and a lockingsleeve 16. The lifting dogs 14 are pivotally mounted to the body 12 andare provided with respective first ends 18 configured for latching aspearhead point therebetween. To this end, the lifting dogs 14 are of asubstantially conventional configuration in which each first end 18 isprovided with a hook-like formation which catches beneath a spearheadpoint (not shown). The locking sleeve 16 is retained on the body 12 andis switchable to locked state where the locking sleeve 16 locks thelifting dogs in the latched position preventing them from pivoting awayfrom each other. In this embodiment the sleeve 16 or at least a portionthereof is seated between or inside the lifting dogs 14 to prevent themfrom pivoting to the ends 18 apart sufficiently to allow a latchedspearhead point to be released. In this way, the overshot 10 ispositively locked on to a spearhead point and, except for a materialfailure, can not be released therefrom. This position is depicted inFIG. 2. The locking sleeve 16 can be switched by an operator to anunlocked state where it is disengaged from the lifting dogs 14 allowingthe ends 18 to be pivoted away from each other in a convention manner toa released position (shown in FIG. 3) where a previously caughtspearhead point can be released from between the lifting dogs 14 in aconventional manner.

[0024] Looking at the components of the overshot 10 in more detail, itcan be seen that the body 12 includes a hollow tubular lower portion 20to which the lifting dogs 14 are pivotally mounted, and an integralreduced diameter upper portion 22. An elongated transversely extendingslot 24 is formed through the body 12. While a majority of the length ofthe slot 24 is in the lower portion 20, an upper end 26 of the slot 24is formed in the upper portion 22. The slot 24 provides room for thelifting dogs 14 to pivot as well as facilitating the assembly of thelifting dogs 14 into the body 12. The outer surface of the lower portion20 is of an octagonal section while the upper portion 22 is formed witha circular section. An upper end of the lower portion 20 tapers to anintermediate portion 28 of the body which is formed of a constantdiameter. The portion 28, is provided with a tapered surface 29 leadingto a stepped face 31 which extends to the upper portion 22.

[0025] Extending radially through the lower portion 20 transverse to theslot 24 is a hole 30 for receiving a pivot pin 32 which pivotally mountsthe lifting dogs 14 to the body 12. In this regard, the pivot pin 32passes through respective eyelets 34 formed intermediate the length ofeach of the lifting dogs 14. A second smaller diameter hole 36 is formedthrough the body 12 parallel to the hole 30 on the side opposite theupper portion 22. The hole 36 seats a spring pin 38 which is disposedbetween the lifting dogs 14 a and 14 b and acts to limit the degree bywhich the ends 18 can pivot toward each other. When the lifting dogs 14are in the latched position, they both bear on opposite sides of thespring pin 38.

[0026] The upper portion 22 is formed with radially extending throughholes 40, 44 and 46, each of these holes extend transversely to theholes 30 and 36. The hole 40 houses a spring 48 and, on opposite sidethereof respective locking balls 50 which together form part of adétente.

[0027] The hole 40 is of a diameter larger than the diameter of thelocking balls 50. The holes 44 and 46 accommodate spring pins 52 and 54.In addition to their function as explained below, the pins 52, 54 alsofacilitate attachment of the overshot to a wireline cable, as is knownin the art.

[0028] Second ends 56 of the lifting dogs 14 are formed with respectiverebates 58 which face each other. As seen most clearly in FIG. 2, in theassembled overshot 10, the ends 56 extend axially beyond theintermediate portion 28 and stepped face 31 of the body 12, and byvirtue of the rebates 58, are spaced radially beyond the surface of theupper portion 22.

[0029] Inwardly directed locating pins 60 are provided at the ends 56inboard of the rebate 58 for seating opposite ends of a compressionstring 62. The compression string 62 is configured to bias the latchingdogs 14 so that the ends 18 are biased toward the capture position.

[0030] The locking sleeve 16 is in the form of a tube of an innerdiameter slightly greater than the outer diameter of the upper portion22 so that the locking sleeve 16 can rotate about the upper portion 22.A lower portion of the locking sleeve 16 is provided with a taperedsurface 64 of reducing diameter leading to constant diametercircumferential wall 66. The wall 66 is provided with diametricallyopposed U-shaped gaps or cut-outs 68. Two first recesses in the form ofradially extending holes 70 are formed in the locking sleeve 16 atlocations that can register with the position of the locking balls 50when the sleeve 16 is in the locked state. Two second recesses 72 (onlyone shown) are also formed in the locking sleeve 16 at positions thatcan register with the locking balls 50 when the sleeve 16 is in theunlocked state. The holes 70 and 72 are of a diameter less than thediameter of the locking balls 50, in this way the sleeve 16 acts toretain the locking balls 50 in the upper portion 22. Thus thecombination of the spring 48, balls 50 and holes 70, 72 act as adétente.

[0031] The locking sleeve 16 is retained on the body 12 by a retainingmember 74 which sits on the upper portion 22 behind the locking sleeve16. The retaining member 74 is in the form of a sleeve and provided withtwo pairs of holes 76 and 78 which are positioned to register with theholes 44 and 46 and through which the spring pins 52 and 54 pass toattach the retaining member 74 to the body 12.

[0032] The unlocked state of the locking sleeve 16 is characterised bythe gap or U-shaped cut-out 68 registering with the upper end 26 of theslot 24, and the ends 56 of the lifting dogs. In this configuration, theends 56 of the lifting dogs 14 are able to move through the gaps 68.Additionally, the holes 72 are in alignment with the holes 40 in whichcase the locking balls 50 partially extend into the holes 72 releasablyholding the locking sleeve 16 in this position. It will be appreciatedthat when the locking sleeve 16 is in the unlocked state, the liftingdogs 14 can be pivoted toward or away from each other to selectivelycapture or release a spearhead point.

[0033] In order to switch the locking sleeve 16 to the locked state, thesleeve 16 is gripped either by hand or by a tool and rotated through90°. As this occurs, the locking balls 50 are pushed inwardly againstthe bias of the spring 48 rolling out of holes 72, and the wall 60 movesbehind or inside the ends 56 being accommodated within the rebates 58.When turned fully for 90°, the holes 70 register with the holes 40allowing the locking balls 50 to be pushed by the spring 48 partiallyinto the holes 70 thereby holding the locking sleeve 16 in the lockedstate. When in this state, the ends 18 of the lifting dogs 14 can not bepivoted away from each other from the latched position to the releaseposition. This direction of pivoting of the lifting dogs 14, if allowed,would be characterised by the ends 56 being pivoted toward each otheragainst the bias of spring 62. However because the wall 60 is disposedwithin the rebates 58, the lifting dogs 14 can not be pivoted in thisdirection. Thus, a spearhead point captured by the lifting dogs 14 ispositively locked and can not be released (except for a materialsfailure) until the locking sleeve 16 is again switched to the unlockedstate.

[0034]FIG. 4 depicts a second embodiment of the lockable overshot 10′.In this embodiment, like reference numbers are used to denote likefeatures with the features which differ from the first embodiment beingdenoted with the addition of a prime symbol. The lockable overshot 10′includes an elongated body 12 of identical configuration and operationto that described in relation to the overshot 10 depicted in FIGS. 1-3.The salient differences between the lockable overshot 10′ and thelockable overshot 10 are as follows. In the overshot 10′ the ends 56′ ofthe lifting dogs 14 a, and 14 b are not provided with the recess 58depicted in FIG. 1. Further, the locking sleeve 16′ has beenreconfigured. In particular, the holes 70, and 72 of the firstembodiment have been replaced with axially extending elongated slots 70′and 72′. As with the previous embodiments, these slots are disposed soas to partially receive the locking balls 50 when the locking sleeve 16′is in the locked state and unlocked state respectively. The slots 70′and 72′ are of a circumferential width smaller than the diameter of thelocking balls to ensure that they cannot escape therefrom. However, theprovision of the slots 70′ and 72′ allows the sleeve 16′ to also slideaxially along the upper portion 22 of the body 12. Further, an end ofthe sleeve 16′ adjacent the lifting dogs 14 has been reconfigured sothat the tapered surface 64′ has been replaced with a stepped surfacewhich leads to a shoulder 69 of constant diameter which in turn stepsdown at edge 71 to circumferential wall 66′. The wall 66′ is tapered soas to reduce in outer diameter in a direction toward the lifting dogs14.

[0035] The retaining sleeve 74 of the first embodiment is now replacedwith a bush 74′ having a tubular body with an increased diameter head,and is attached to the end of the upper portion 22 distant the lowerportion 20 by any conventional means (not shown). A spring 75 is locatedover the tubular body of the bush 74′ seated between the locking sleeve16′ and the increased diameter head of the retaining bush 74′. Thetubular body of the retaining bush provides an upper limit to thesliding motion of the locking sleeve 16′

[0036] By virtue of the taper of the wall section 66′, the provision ofslots 70′ and 72′ and spring 75, the sleeve 16′ is able to slide axiallyalong the body 12 as well as rotate about the axis of the body 12. Whenthe locking sleeve 16′ is in the position as depicted in FIG. 4, thewall 16′ is disposed inside the ends 56′ of the lifting dogs 14. Thelocking balls 50 (not shown in FIG. 4) extend partially into the slots70′ and act as a détente to assist rotational motion of the lockingsleeve 16′. The spring 75 biases the locking sleeve 16′ axially towardthe lifting dogs 14 so that the wall 66′ abuts against the inside of theends 56′. This effectively prevents the first ends of the lifting dogs14 from pivoting away from each other to release a previously capturedspearhead point.

[0037] By rotating the locking sleeve 16′ through ninety degrees so thatthe locking balls 50 register with the slots 72′, the sleeve 16′ isswitched to the unlocked state where the ends 56′ are located withinrespective gaps 68. This corresponds with the unlocked state where thefirst ends 18 of the lifting dogs 14 can be pivoted away from eachother.

[0038] By providing a taper on the wall 66′, which terminates at edge 71of the shoulder 69, the locking sleeve 16′ also provides a visualindication of the wear of the lifting dogs 14. As the lifting dogs 14wear, the wall section 66′ is able to be pushed further in the axialdirection by the springs 75 thereby reducing the distance between theends 56′ and the shoulder 69, when the locking sleeve 16′ is in thelocked state. The diameter of the wall section 66′ adjacent the edge 71can be produced to a specific diameter which, when adjacent the ends 56′provides an indication that the lifting dogs 14 are worn out and shouldbe replaced.

[0039] Now that an embodiment of the present invention has beendescribed in detail it will be apparent to those skilled in the relevantarts that numerous modifications and variations may be made withoutdeparting from the basic inventive concepts. For example, the retainingmember 74 may be attached to the body 12 by means other than the springpins 52 and 54, for example by use of a bolt or by way of screw threadedengagement between the sleeve 74 and upper portion 22. Also, other formsof mechanism other than the détente system formed by the spring 48locking balls 50 and holes 70, 72 can be used to releasably hold thelocking sleeve 16 in the locked or unlocked states. Additionally, thelower portion 20 of the body 12 can be made of a transverse sectionother than octagonal, for example, circular, square or hexagonal. Allsuch modifications and variations are deemed to be within the scope ofthe present invention, the nature of which is to be determined from theabove description.

1. A lockable overshot including at least: an elongated body; at leasttwo lifting dogs pivotally mounted to said body, said lifting dogshaving respective first ends configured for latching a spearhead pointtherebetween, and said first ends pivotally moveable toward each otherto a latched position wherein a spearhead point is latched therebetweenand pivotally moveable away from each other to a release position wherea latched spearhead point is released; and, a locking sleeve retained onsaid body and switchable between: a locked state where said lockingsleeve locks said lifting dogs in said latched position and, an unlockedstate where said locking sleeve allows said lifting dogs to move to saidrelease position.
 2. The lockable overshot according to claim 1 whereinsaid lifting dogs have respective second ends opposite said first endsand said lifting dogs are pivotally mounted at a location intermediatesaid first and second ends to said elongated body, and at least aportion of said locking sleeve is disposed inside said second ends whenin said locked state.
 3. The lockable overshot according to claim 1wherein said locking sleeve is rotatably retained on said elongated bodyand is switched between said locked state and said unlocked state byrotating said locking sleeve about a longitudinal axis of said elongatedbody.
 4. The lockable overshot according to claim 1 wherein said lockingsleeve is provided at one axial end with a circumferential wall formedwith a gap for each of said lifting dogs wherein when in said lockedstate said wall is located inside said respective second ends and whenin said unlocked state said second ends are located in said gaps.
 5. Thelockable overshot according [[1-4]] claim 4 wherein said lockableovershot further includes détente means for releasably holding saidlocking sleeve selectively in said locked and unlocked states.
 6. Thelockable overshot according to claim 5 wherein said détente meansincludes a biased locking ball extending from an outer surface of saidelongated body and first and second recesses formed in said lockingsleeve for partially receiving said locking ball, said first recessdisposed to register with said locking ball when said locking sleeve isin said locked state and said second recess disposed to register withsaid locking ball when said locking sleeve is in said unlocked state. 7.The lockable overshot according claim 5 wherein said circumferentialwall is of constant outer diameter.
 8. The lockable overshot accordingto claim 4 wherein said circumferential wall is formed with a reducingouter diameter in a direction toward said lifting dogs.
 9. The lockableovershot according to claim 8 wherein said lockable overshot furtherincludes bias means for biasing said locking sleeve axially in adirection toward said lifting dogs.
 10. The lockable overshot accordingto claim 9 wherein said first and second recesses are in the form ofaxially extending slots.
 11. The lockable overshot according to claim 10wherein said locking sleeve is further provided with a shoulder ofconstant outer diameter adjacent said circumferential wall on a sidedistant said lifting dogs.